When you are in any cave, then you can't access the world map. If you are in a multi-map cave, then the map function shows you just the cave maps (that you have found so far). If the cave is a single map, the map function is disabled entirely.

Why not show the single map anyway? I think it's just to save work that yields limited benefit - each of those multi-map caves requires some extra setup.

Like I said, if the cave is only one map, then the function doesn't work. You get toast telling you that the map is unavailable. This is not a bug, it's a feature. Whether we want to change that feature is a different question.

Edit:clarification/correction. If the cave is only one map *per level*, then the map function is turned off and you get the toast. I believe, for instance, that the wolf cave, which is one map for each of two levels, does not display. But the snake cave, which has three maps all on one level, should display them. Need to double check that.